(1) Field of the Invention
The present invention relates generally to a temperature control system of an exhaust gas component sensor for sensing the concentration of a component such as oxygen (O.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), hydrocarbon (HC) or nitrogen oxide (NO.sub.x) contained in exhaust gases produced by an internal combustion engine for a vehicle which concentration is closely related to the air-fuel ratio of an air-fuel mixture burned in the engine or the overall air-fuel ratio of the engine.
(2) Description of the Prior Art
As is well known in the art, as one of the exhaust gas component sensor, for example, an oxygen sensor is formed of a body of zirconium dioxide which is exposed on one surface to a reference oxygen concentration and exposed on a second, opposed surface to an oxygen concentration to be sensed and may, in the presence of thin porous electrically conductive coatings on the two surfaces, generate an electrical potential between the two surfaces which is indicative of the oxygen concentration of the surface to be sensed.
When the sensors are inserted into the exhaust system of an internal combustion engine, by coating the surface of the zirconium dioxide body which is exposed to the exhaust gases with a catalytic metal material, such as platinum, the sensor will generate a relatively large output signal whenever the combustion mixture for the internal combustion engine has been at an air/fuel ratio less than a predetermined air-fuel ratio such as, for example, the stoichiometric mixture ratio and will generate a relatively low signal whenever that mixture has been prepared with an air/fuel ratio greater than the predetermined air/fuel ratio. In other words, the sensor will generate an output signal having a virtual step function, going from high to low at the predetermined air/fuel ratio, for increasing air/fuel ratios. However, when such a sensor is inserted into the exhaust system of an internal combustion engine, it is greatly influenced by the heat of exhaust gases of the engine. When the exhaust gas component sensor is excessively heated, the durability of the sensor is reduced.
Also, the sensing performance of the exhaust gas component sensor is varied in accordance with the temperature thereof. For example, the output of the sensor is relatively low and high when the temperature of the sensor is relatively low and high, respectively at the same air-fuel ratio as shown in FIG. 1 of the accompanying drawings which illustrates the case of the air-fuel ratio A/F being about 14.0:1.
Accordingly, it is necessary to control the temperature of the exhaust gas component sensor to an optimum value for protecting the sensor and stabilizing the output of the sensor. Such a necessity is specially made conspicuous when an exhaust gas component sensor is employed in an air-fuel ratio control system which controls the air-fuel ratio of an air-fuel mixture burned in an engine to a desired air-fuel ratio optimum to operating conditions of the engine in accordance with the relationship between an air-fuel ratio sensed by the sensor and the desired air-fuel ratio.
As factors having influence on the temperature of the exhaust gas component sensor, there is existent both heating due to the heat of the engine exhaust gases and cooling due to the flow of surrounding atmospheric air produced due to travelling of a vehicle equipped with the engine. Since when the engine is running at medium and high speeds, the temperature of the engine exhaust gases is relatively high as shown in FIG. 2 of the accompanying drawings, it is necessary to cool the exhaust gas component sensor by surrounding atmospheric air, while when the engine is idling and running at low speeds, since the temperature of the engine exhaust gases is relatively low as shown in FIG. 2, it is unnecessary to cool the sensor but it is necessary to maintain or to increase the temperature of the sensor.